In chemical technology it is often necessary to remove from a mixture of gases a particular gas or group of gases. It can be particularly difficult to effect such a separation when the gas or gases to be removed constitutes a large fraction of the mixture. One example of such a gas mixture is waste biogas generated by a refuse landfill, which is typically composed principally of methane and carbon dioxide, with from about 30 to about 50 percent of the mixture being carbon dioxide. To convert such a waste biogas to a methane fuel or chemical feedstock, as much of the carbon dioxide should be removed as feasible.
An article by S. Kimura and G. E. Walmet in Separation Science and Technology, volume 15, pages 1115-1133 (1980) discloses a technique for purifying biogas mixtures and other gas-separation processes which employs an "immobilized liquid membrane." The immobilized liquid membrane technique employs a flat, porous-membrane assembly. To purify a biogas by the immobilized liquid membrane technique, an aqueous solution of potassium carbonate and cesium carbonate is applied to a flat, porous cellulose membrane to form an immobilized liquid membrane. The cellulose membrane is hydrophilic and defines a feed side of the membrane assembly. The hydrophilic cellulose membrane is supported on a porous polypropylene membrane, which is hydrophobic. The polypropylene membrane is in turn supported by a metal screen, which defines a sweep side of the membrane assembly. A stream of the biogas is passed over the feed side of the assembly and a stream of air is passed over the sweep side of the assembly. Both of the gas streams must be kept properly humidified to prevent the drying out of the liquid membrane on the one hand or the flooding of the membrane on the other.
The permeability of methane through the aqueous immobilized liquid membrane disclosed in the Kimura and Walmet article is relatively low, while that of carbon dioxide is relatively high. The flux of carbon dioxide through the immobilized liquid membrane is facilitated by certain reactions of carbon dioxide with the water and various ionic species in the liquid of the liquid membrane. The transport of carbon dioxide through the membrane due to chemical reactions enhances the normal permeation rate of carbon dioxide through the liquid without any chemical reaction. The following reversible reactions take part in the selective transport of carbon dioxide across the liquid membrane: EQU CO.sub.2 +H.sub.2 O.revreaction.H.sup.+ HCO.sub.3.sup.- EQU CO.sub.2 +OH.sup.- .revreaction.HCO.sub.3.sup.- EQU HCO.sub.3 .sup.- .revreaction.H.sup.+ +CO.sub.3.sup.-2.
These reactions together with direct solubility effects give rise to a net dissolution of carbon dioxide on the feed side of the membrane assembly where the partial pressure of the carbon dioxide is relatively high and a net regeneration of carbon dioxide on the sweep side of the assembly where the partial pressure of carbon dioxide is relatively low. The flow of air on the sweep side of the membrane assembly maintains a low partial pressure of carbon dioxide on that side, which tends to increase the degree of separation between the methane and carbon dioxide in the biogas.
An article by S. Kimura et al. in Recent Developments in Separation Science (CRC Press, Cleveland, Ohio, 1978) discloses that the facilitated permeation rate of carbon dioxide through aqueous carbonate solutions is influenced by the rate of diffusion of ionic species across the liquid membrane, as well as by the rates of the various reactions in the liquid membrane. Thus it is advantageous to minimize the thickness of the membrane to minimize the time it takes for ionic species to diffuse across the membrane. However, conventional porous cellulose films for liquid membrane applications are no thinner than about 46 .mu.m, since thinner films are too fragile to handle. Porous polysulfone films, which are similarly employed in conventional immobilized liquid membranes, are also no thinner than about 46 .mu.m. It would be advantageous, however, to achieve higher flux rates than can be achieved with the immobilized liquid membrane technique using conventional porous cellulose and polysulfone films.
The flat geometry of conventional immobilized liquid membrane systems gives rise to additional problems. The membrane surface area per unit equipment volume for flat immobilized liquid-membrane systems is limited, which generally leads to high equipment cost.
Since typically from about 30 to about 50 percent of landfill biogas is carbon dioxide and almost complete removal of the carbon dioxide is desired, large volumes of gas must be brought into contact with the immobilized liquid membrane. Such large volumes of gas make it difficult to maintain an optimum level of humidity on both sides of the membrane and consequently the liquid membrane frequently tends to flood or dry out. Kimura and Walmet in fact concluded that the problems of humidity control could make the operation of an immobilized liquid membrane system difficult, if not impractical.